Wavelet transform,STFT, freq. localization

taken a signal composed of 4 spectral components, with Fourier angular frequencies w1<w2<w3<w4.

Take now a completely different signal, a chirp, made of the same 4 spectral components.
IT is said that if the chirp increases in frequency with time (meaning its instantaneous frequency), those 4 spectral, Fourier components will not be present in the signal all at the same time.
If the signal last 40 second, it is possible that the first 10 s are dominated by the lower frequency w1, and the last 10 second by the frequency w4......
They speak of frequency localization in time......

I would instead say that, also in the case of the chirp, those 4 spectral components are present the whole time, for the whole duration of the signal.....

Any clarification on frequency localization please...

wavelets and STFT are used to find the local spectral content in that case.....